Page 2 - Iannucci_alii_2015

GNGTS 2015 SESSIONE 3. 2

This paper reports the preliminary results of a geophysical campaign of seismic noise
measurements carried out in May 2015 at Cala Rossa Bay. In recent years, several studies
applied ambient noise techniques to investigate landslide-involved slopes (Del Gaudio et
al., 2008; Burjanek et al., 2010, 2012) orto characterize blocks of unstable cliffs (Got et al.,
2010; Panzera et al., 2012; Galea et al., 2014), through different approaches among which HN
spectral ratios, f-·k analysis, site to reference spectral ratios, polarization analysis, base noise
level variations.
The noise measurement stations were distributed in conespondence of the unstable sea cliff
as well as on the stable plateau area (i.e. located about 100 m far from the cliff). The seismic
noise was recorded for l hour and was analyzed by computing the Fast Fourier Transfonn
(FFT) of the three ground-motion components as well as the HVSR. Because of the sirnilar
geologica! and geomorphological setting, the results obtained for the Cala Rossa Bay test site
were compared to the ones obtained forAnchor Bay (Malta), reported in Galea et al. (2014) and
partially reprocessed for the here discussed comparison.
The goal of this research is to previde new evidences about local seismic response in
unstable sea cliffs through the analysis of single-station noise records. The relevance of such
a geophysical application consists on the possibility to manage the hazard related to sea-cliff
landslides in order to mitigate the associateci coastal risk.
Geologica! ami geomorphological setting. The Favignana Island belongs to the Egadi
Archi pelago and is part of the NW sector of Sicily (Italy), which represents the emerged western
edge of the Sicilian-Maghrebian Chain, originated from the deformation of the Meso-Cenozoic
Northern African continentalmargin (Scandone et al., 1974; Antonioli et al., 2006). Tectonic
units, overthrusted in the Middle Miocene and Lower Pliocene, compose the Egadi lslands as
well as the whole Trapani area (Nigro et al., 2000). The Favignana Island is mainly built-up of
Mesozoic-Lower Tertiary car·bonate deposits, covered by trar1sgressive Plio-JPleistocene shallow-
water marine deposits (Abate et al., 1995, 1997; Catalano et al., 1996). The Lower Pleistocene
deposits widespread outcrop along the eastern slope of the emerged paleo-Favignana Island in
a pull-apart basin, probably generateci by transtensional faults activity (Slaczka et al., 2011).
In Cala Rossa Bay, located in the eastern portion of the Favignana Island, porous car·bonate
grainstones of the Lower Pleistocene widely outcrop, about 20 m thick ar1d with almost-
horizontal strata; these carbonate rocks overlie high-plasticity clays of Pliocene age with a
thickness varying from 5 up to 10m (Fig. la). The clays overlie massive carbonate grainstones
ofthe Lower-Middle Miocene, with a thickness varies between 10 and 30 m (Abate et al., 1995,
1997; Tondi et al., 2012), which does not outcrop at Cala Rossa Bay.
This geologica! setting characterized by the over-position of stiff rocks on a more plastic
substratum (Goudie, 2004) leads to a lateral spreading phenomenon which widely involve
the sea cliff. More in particular·, the horizontal defonnations affecting the clayey materials,
with a visco-plastic behavior, induce cracks ar1d failures in the overlying stiff rock causing the
detachment of single rock blocks and the generation of falls and topples. Lateral spreading
processes act selectively, shaping plateau of stiff rocks bordered by unstable cliffs. Lateral
spreading phenomena represent a gravity-induced instability highly dar1gerous respect to the
coastal cultura! heritages and cause several touristic places in the Meditenanean ar·ea (Canuti et
al., 1990; Gigli et al., 2012; Falconi et al., 2015) to be inaccessible or avoided for tourists.
The study ar,ea. In the western sector of Cala Rossa Bay in Favignana Island the ongoing
lateral spreading produced a complex ground crack pattern which favors gravitational
instabilities also inftuenced by the presence of ar1 extensive network of nnderground tunnels
related to historical quar1y activities in the porous carbonate grainstones and terminated in the
latter half of the last century (Falconi et al., 2015).
Based o n fie l d surveys, a main gravitational joint system was devised, consisting of three open
fractures whose direction is nearly parallel t o the coastline (Fig. l b). Three different unstable
zones, separated by the three opened ground cracks, and a fourth stable zone, consisting in the

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